1. Field of the Invention
The present invention relates to a process for producing a dielectric ceramic, and in particular to a process for producing a dielectric ceramic having a desired temperature property of resonant frequency, without change in the composition, and suited to use for high frequency.
2. Description of the Prior Art
Temperature property of resonant frequency of dielectric ceramics (which may be hereinafter called merely "temperature property of dielectric ceramics") is generally controlled by changing the composition thereof, that is, changing the kinds or contents of constituent elements thereof. In particular, dielectric ceramics used in the micro wave band are required to have a high relative dielectric constant and unloaded Q and a good resonant frequency temperature property. The dielectric ceramics with a composition that satisfy such requirements include for example ceramics of a Ba(Zn Ni).sub.166 (Ta,Nb).sub.170 O.sub.3 system having a perovskite-type complex structure.
In the ceramic of the above composition, a part of Zn ions are replaced with Ni ions for improvement in the temperature property. This replacement acts so as to reduce the relative dielectric constant and unloaded Q. In order to compensate these reduction, Ta ions are partially replaced with Nb ions in attempt to improve the relative dielectric constant. However, this results in deterioration of the temperature property and unloaded Q. As described above, only change in composition can not improve all of the properties required. In addition, it causes the problem that the production unit used needs to be cleaned thoroughly before production, every time the composition is changed.
The U.S. Pat. No. 4,487,842 of Nomura et al. discloses a process comprising the steps of adding a small amount from 0.5 to 5 mol % of Mn to a perovskite type structure compound oxide powder consisting of at least one member selected from the group consisting of Ba(Zn.sub.166 Ta.sub.170)O.sub.3 and Ba(Mg.sub.166 Ta.sub.170)O.sub.3, followed by calcination, and then sintering the Mn-treated oxide powder at 1,550.degree.-1,600.degree. C. In this process, to control the temperature property of the dielectric ceramic to be obtained, the composition of the ceramic must be changed. Further, the process is disadvantageous in that Mn which is added to promote sintering to produce a dense ceramic in the process is a element that does not constitute a perovskite structure, so that it may lower the dielectric properties of the dielectric ceramic obtained, and that the addition of Mn makes process control complicated.
The U.S. Pat. No. 4,121,941 of Kawashima et al. discloses, e.g., a method of making a low microwave loss ceramic of the formula: EQU w(3BaO.ZnO.Nb.sub.2 O.sub.5)-x(3BaO.ZnO.Ta.sub.2 O.sub.5)
wherein 0.01.ltoreq.w.ltoreq.0.99, 0.01.ltoreq.x.ltoreq.0.99, and w+x=1, in solid solution, said method comprising the steps: calcining a mixture of the oxides or the like of the metals to be contained in the ceramic, and hot-pressing at a pressure of at least 50 kg/cm.sup.2, at 1,100.degree. C. to 1,450.degree. C. for at least 30 minutes. To regulate the temperature property of the ceramic to be obtained in this method, the composition of the ceramic must be changed. This method does not needs the addition of Mn but requires hot-pressing to obtain a dense ceramic. The hot-pressing is disadvantageous in that it requires a special unit, that it is not suitable for mass production, and that it requires demolding operation after sintering which makes the manufacturing process complicated.